1. Field of the Invention
The present invention relates to a high-strength martensitic stainless steel having excellent anti-stress corrosion cracking property and a method for making the same, and more particularly to a high-strength martensitic stainless steel showing excellent anti-stress corrosion cracking property in an environment containing CO.sub.2 and H.sub.2 S in such a case of drilling and transporting crude oil and natural gas, and a method for making the same.
2. Description of the Related Arts
Crude oil and natural gas recently extracted often contain large amounts of CO.sub.2 and H.sub.2 S. To cope with this, martensitic stainless steels such as 13Cr stainless steel are adopted instead of conventional carbon steel.
Ordinary martensitic stainless steels, however, have superior corrosion resistance to CO.sub.2 (hereinafter referred to simply as "corrosion resistance") but have insufficient stress-corrosion cracking resistance to H.sub.2 S (hereinafter referred to simply as "anti-stress corrosion cracking property"). Accordingly, a martensitic stainless steel having improved anti-stress corrosion cracking property while maintaining favorable strength, toughness, and corrosion resistance has long been wanted.
Materials which satisfy the requirements of strength, toughness, and corrosion resistance, and also of anti-stress corrosion cracking property are disclosed in Examined Japanese Patent Publication No. 61-3391, Unexamined Japanese Patent Publication No. 58-199850 and 61-207550. Those materials show a resistance to an environment containing only a slight quantity of H.sub.2 S, but they generate stress-corrosion cracking in an environment at over 0.01 atm. of H.sub.2 S partial pressure. So those materials can not be used in an environment containing a large amount of H.sub.2 S.
On the other hand, some of martensitic stainless steels which have an improved anti-stress corrosion cracking property in an environment exceeding 0.01 atm. of H.sub.2 S partial pressure are introduced. Examples of that type of martensitic stainless steel are disclosed in Unexamined Japanese Patent Publication Nos. 60-174859 and 62-54063. Those materials are, however, also unable to completely prevent stress corrosion cracking caused by H.sub.2 S. From the viewpoint of strength, a trial for improving the strength on all the martensitic stainless steels described above resulted in a significant degradation of their toughness and anti-stress corrosion cracking property. Accordingly, all those martensitic stainless steels have an unavoidable problem in that either toughness or anti-stress corrosion cracking property is sacrificed. As a result, those martensitic stainless steels can not be used as a deep OCTG (Oil Country Tubular Goods), for example, for which a high strength, anti-stress corrosion cracking property, anti-corrosion property, and toughness at the same time is requested.